Step 1: Identify the subject. 'Everyone' is treated as singular in the main clause but plural (they) in the question tag. Step 2: Identify the verb. The verb is 'was' (to be). Since the tag pronoun is 'they', the plural form 'were' must be used. Step 3: Check for polarity. The main clause is positive, so the tag must be negative: 'weren't they'.